1. Field of the Invention
The invention relates to a spotlight, in particular for at least partially illuminating a stage, wherein the spotlight comprises a housing for accommodating a light source and a Fresnel step lens, through which light emitted by the light source can leave the housing when the light source is accommodated in the housing.
2. Background of the Invention
Such spotlights are used nowadays, for example, for illuminating stages. A sufficiently intense light source which can be a gas discharge lamp or a halogen lamp, for example, is arranged in a housing provided for this purpose. The use of LEDs is also conceivable. The housing is usually nontransmissive to light apart from an opening covered by the lens and is possibly equipped with reflectors in the interior. The aim is to direct as much of the light emitted by the light source onto the area to be illuminated as possible. For this purpose, use is made of a lens which focuses the light emitted by the light source. In this context, the use of a Fresnel step lens here has been known for some time. This type of lens is particularly advantageous in the case of large and in particular thick lenses which therefore have a short focal length in comparison with their size. A short focal length is expedient for the application in a spotlight in order to be able to keep the physical dimensions of the spotlight as small as possible. The light source needs to be arranged in the vicinity of the focal point of the lens used, with the result that a short focal length results in a physically short spotlight.
However, this results in an increased thickness of the lens. In order in this case to be able to save weight and material, a Fresnel step lens is used instead of a normal lens. In the case of such a lens, the glass body is divided into ring-shaped regions, in which the thickness is reduced individually, with the result that a Fresnel step lens has a large number of ring-shaped steps. However, the lens retains its focal length and therefore its essential optical property. However, the imaging quality is generally impaired by the step structure in comparison with a lens consisting of a solid material.
In principle, Fresnel step lenses can have steps on both sides or else only on one side. In most applications today, preferably Fresnel step lenses with steps on one side are used since they can be manufactured more easily and also have optical advantages. In this case, in particular that side of the lens which is remote from the light source is provided with steps, while the inner side, i.e. that which faces the light source arranged in the interior of the housing, is smooth. If steps are also provided on that side of the lenses which faces the light source, this results in a reduction in the efficiency of the lens.
With such a spotlight, a light spot with a specific shape, for example a circular shape, which has a homogeneous brightness distribution is intended to be achieved. For this purpose, that side of the lens which faces the light source is generally roughened, which is conventionally referred to as stippling. This can take place as early as during the pressing operation of the lens, for example, or by means of subsequent sandblasting of a surface which is initially pressed smooth.
In the case without such stippling, it would be possible for the light-emitting means of the light source, for example the filaments of a halogen lamp, to be imaged with high definition in the imaging plane if it is located at a distance in front of the lens that is slightly greater than the focal length of the lens. This would result in optical imaging of the contour of the illuminating part of the light-emitting means in the viewed plane which has greater or less definition being produced instead of the desired round light spot for illuminating a region on a stage. This is disruptive and undesirable.
In order to avoid this, the Fresnel step lenses used in spotlights are roughened on the smooth surface in such a way that precise focusing and therefore high-definition imaging is no longer possible. In the case of a so-called profile spotlight, initially an intermediate image is generated in the spotlight, which is limited in terms of its lateral extent there with the aid of a usually circular intermediate diaphragm, in particular an adjustable iris diaphragm, and is then projected as a circular light spot with a very sharp edge in the stage region in the imaging plane.
One disadvantage with the stippling is, however, that an additional working step in the form of a subsequent sandblasting process is required for this purpose or the stamping part in question is given a corresponding stippled surface contour during pressing of the lens. In this case, the surface roughness should also not be too great since the rays of light should only be deflected slightly. Otherwise, the light scattering at the roughened surface would be so great in terms of its effect that the lens would lose efficiency overall. This loss in efficiency is particularly pronounced since the stippling is located on that side of the lens through which radiation passes first, i.e. the side facing the light source. As it passes through this roughened surface, the light is scattered to such an extent that, to a small extent, said light no longer emerges on the other side of the Fresnel step lens or reaches the steps. Instead, the light enters the region of the steep flanks and the very rounded transition region between the step faces and the flank faces. This light is broken away in an uncontrolled manner in a wide variety of directions and no longer makes a contribution in the region of the desired light field on the stage. This effect is greater the greater the stippling.